This invention related generally to the digital manipulation of a continuous time domain sample that is to be sampled in a digital oscilloscope, and more particularly to a digital filter that is capable of increasing the bandwidth of the sampling system beyond the bandwidth range achievable in an analog system.
The present state of the art deals with an attempt to increase bandwidth based upon the assumption that only analog manipulation techniques for modifying a signal to improve the bandwidth characteristics of an apparatus are possible. Other digital techniques are seen as manipulations of the signal that change the output result of the system. This results in a design methodology in which analog design engineers painstakingly design to the best of their ability analog circuitry that has high bandwidth, flat frequency response, good pulse response and is noise-free.
In many cases, these designs are extremely complicated, particularly in the design of a digital oscilloscope. Some reasons for this difficulty are:
1. There is a wide variation in the gain of the front-end, sometimes approaching 60 dB of dynamic range.
2. The signal is typically distributed to multiple Analog/Digital Converters (ADCs) in order to boost sample rate. This distribution is through buffers that tend to decrease the bandwidth of the system.
3. High bandwidth/high sample rate oscilloscopes typically use the absolute best of present electronics technology and are therefore being pushed to the limits of the hardware.
4. Bandwidth, noise, and pulse-response are a set of conflicting requirements that must be reconciled.
What makes things worse is that even upon observing and confirming the existence of problems with the bandwidth, flatness, pulse-response, and noise performance in the system, little can often be done to rectify the situation. This is because circuits designed to fix such problems are often not practically realizable.
Therefore, it would be beneficial to provide an improved Digital Signal Processing (DSP) method and apparatus capable of surgically dealing with lack of bandwidth, while offering some additional control of the pulse-response and flatness, and thereby decreasing the overall noise of the system as well.
It is therefore an object of the invention to provide an improved Digital Signal Processing (DSP) method and apparatus capable of surgically dealing with lack of bandwidth, while offering some additional control of the pulse-response and flatness.
Another object of the invention is to provide an improved Digital Signal Processing (DSP) method and apparatus capable of surgically dealing with lack of bandwidth, while offering some additional control of the pulse-response and flatness, and in which the overall noise of the system can be decreased.
A still further object of the invention is to provide an improved Digital Signal Processing (DSP) method and apparatus capable of surgically dealing with lack of bandwidth, while offering some additional control of the pulse-response and flatness by increasing the bandwidth in a very controlled manner.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and the drawings.
Generally speaking, in accordance with the invention, consider a system whose frequency response is shown in FIG. 2. As is apparent in FIG. 2, the response violates the limits on a specified 2 GHz frequency response by dropping below anindicated xe2x88x923 dB point prior to 2 GHz. In other words, this unit is specified to be a 2 GHz unit, but the bandwidth provided does not provide an adequate response clearly out to the 2GHz limit. However, the system nearly makes itxe2x80x94the signal is only attenuated by around 4 dB out at 2 GHz.
What this system needs in order to meet its bandwidth specification is a slight boost in the frequencies in the range of 1.8 to 2 GHz. While such a boost might appear to be an easy solution, this is quite a difficult problem to solve in hardware. First, the frequencies of the hardware are quite high. Second, it is difficult to increase only the frequency response in this small band without ruining the performance of the unit.
Therefore, in accordance with the invention, a filter as specified in FIG. 3 is required. Such a filter is flat out to 1.8 GHz with 0 dB gain. At 1.8 GHz, the gain rises log-linearly to 4 dB. The filter is unspecified out to 3 GHz after which an attenuation of xe2x88x925 dB is specified. The exact reasons for some of these specifications will be described below. These specifications are flexible in the filter design, and may be customized for any number of various design requirements.
After applying a filter with a response characteristic as shown in FIG. 3, the frequency response of the overall system would become that of FIG. 4. Notice that the frequency response up to 1.8 GHz is unaltered and that the system achieves 0 dB gain at exactly the bandwidth desiredxe2x80x94that of 2 GHz. In other words, there is 3 dB margin at the specified bandwidth of the system.
Therefore, the invention focuses on providing a filter that meets the specifications shown in FIG. 3 and therefore surgically boosts the frequency response of the system with no other adverse affects.
In addition to increasing the bandwidth of the system to 2 GHz, the filter in accordance with the invention has an effect on the noise floor and roll-off. As is shown in FIG. 5, the noise floor of the boosted system is 5 dB below the un-boosted system noise floor because of the attenuation added outside the pass-band. Furthermore, the roll-off rate is approximately the same as in the un-boosted system.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combination(s) of elements and arrangement of parts that are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.